WO 2012/056 275 A1 discloses a machine combination comprising an internal combustion engine and a generator for charging the battery of a hybrid drive. The internal combustion engine is designed as a pair of cylinder-piston units, wherein each of the two pistons is in driving engagement with its own crankshaft via connecting rods. The generator arranged in series with the cylinder-piston units is driven by the crankshafts.
One object of the invention is to improve a power unit of the design mentioned in the introduction so that a particularly quiet—low-noise and vibration-free—functioning both of the engine and of the unit as a whole can be achieved, and, besides a particularly simple and particularly space-saving structure, a design with low friction losses and low susceptibility to wear can be achieved in addition, which is particularly suitable for the interval operation of a hybrid drive. A further object of the invention is to specify a vehicle comprising a power unit of this type.
This object is achieved in accordance with the invention by the subject matter of claim 1 in respect of the power unit and by the subject matter of claim 4 in respect of the vehicle.
The invention is based on the principle of specifying a power unit, in particular for a hybrid vehicle, comprising a reciprocating piston engine and comprising a generator which can be in driving engagement therewith. The reciprocating piston engine has at least two pistons, which are guided in at least two cylinders in tandem arrangement. The reciprocating piston engine also has two counter-rotating crankshafts, which are connected to the pistons by connecting rods and which are mechanically coupled so as to be in phase. In accordance with the invention a first generator can be driven by the first crankshaft and a second generator can be driven by the second crankshaft.
The use of two generators opens up new possibilities for the balancing free mass forces and vibratory torques so that the smoothness of the piston engine is improved. Smaller generators can also be used.
The generators are preferably connected to the respective crankshafts by a toothed gear mechanism or by a traction means. The connection by a traction means, in particular a traction means which comprises teeth on either side, improves the low-noise operation.
In a preferred embodiment the first generator and the second crankshaft rotate in opposing directions. The second generator and the first crankshaft also rotate in opposing directions. The generators and the crankshafts rotate at a rotational speed ratio of 2:1. The generators each have at least one imbalance and thus serve as balancing shafts. This embodiment has the advantage that the generators assume a dual function. On the one hand they serve to supply power. On the other hand the generators balance second-order mass forces. The generators thus act in accordance with the principle of a Lanchester balancer. If two counterweights are each provided as imbalance, these can be of equal weight or different weight depending on the position of the generator so as to balance an undesired moment about the longitudinal axis of the engine.
In a development of this embodiment each generator has two counterweights, which are distanced from one another in the longitudinal direction of the generator. Specifically, the two counterweights are attached at either axial end of the generator. The two counterweights are each of equal weight. Undesirable moments are balanced by this arrangement.
There are various possibilities for the coupling of the generators to the respective crankshafts. By way of example, the toothed gear mechanism can comprise a toothed crank web, which is engaged or coupled to a drive pinion of the relevant generator. The toothed gear mechanism can alternatively comprise at least one toothed gear, which is connected, for conjoint rotation, to a shaft pin of the corresponding crankshaft, said shaft pin protruding over the main bearing, and which is coupled to a drive pinion of the relevant generator. A chain or a toothed chain or a toothed belt are potential traction means, and are driven by sprockets.
In a preferred embodiment a first traction means connects the first generator to the first crankshaft and a second traction means connects the second generator to the second crankshaft. The traction means are relatively short in this embodiment, which facilitates the guidance thereof.
The generators are preferably arranged on either side adjacent to the crankshafts, wherein the crankshaft axes span a virtual reference plane through the main bearing. The generator axes can be arranged either in the reference plane or outside the reference plane, more specifically between the crankshafts and the upper dead centre of the pistons. The arrangement of the generator shafts in the reference plane or outside the reference plane leads to advantages with regard to the position of installation of the power unit in the engine. The arrangement of the generator axes above the reference plane additionally has the advantage that the engine is relatively narrow.
The crankshafts preferably have counterweights, which are arranged so as to be phase-shifted, so as to balance free mass forces.
In addition—also so as to achieve a more compact design and a reduction of friction losses—the cylinders can be arranged with an inwardly directed offset so that they have a shorter distance from one another compared to that corresponding to the distance between the crankshafts. The offset of the cylinders thus runs in the direction of the anti-thrust side. As a result, the connecting rods at the time of the greatest application of force thus have a somewhat steeper position with respect to the piston/cylinder axis, such that lower forces influencing the piston friction and the cylinder wear occur at the piston or at the cylinders, accordingly.
It is particularly advantageous to operate the reciprocating piston engine with a firing interval of 0° crankshaft angle in 4-stroke operation.
For the synchronisation of the crankshafts it is advantageous that these have toothed crank webs, which are in engagement with one another. Alternatively, a traction means which comprises teeth on either side, in particular a toothed belt which comprises teeth on either side, can be provided for the synchronisation, which traction means couples the two crankshafts to one another, wherein a first side of the toothed belt is in engagement with the first crank web and a second side, in particular a back of the toothed belt, is in engagement with the second crank web. The use of the traction means damps the development of noise. So as to reduce the loading of the tooth pairs, it can be provided that the toothed belt which comprises teeth on either side is wrapped around the first crank web, wherein the toothed belt on either side has 1 to 5 teeth, in particular 4 teeth, more than the toothing of the second crank web. The additional number of teeth allows the toothed belt to be lifted off and allows the toothed gear to be overtaken within the raised toothed belt. For this purpose, the toothed belt should rise radially beyond the addendum circle of the toothed gear by more than the dedendum. The centrifugal force supports this lifting. The motivation for this is the avoidance of the recurrence of the maximum loading on the same teeth of the toothed belt which comprises teeth on either side, for example with a transfer of the firing force in the case of 4-stroke operation with 360° firing interval.
In an alternative embodiment the crankshafts can be coupled so as to be in phase by means of toothed gears, which are in engagement with one another, wherein the toothed gears are arranged between the main bearings of the crankshafts or on the shaft pins of the crankshafts, said shaft pins protruding over the respective main bearings. The toothed gears can be spur gears having straight teeth or helical teeth or tensioned toothed gears (scissor gears or hunting gears). In the case of an embodiment of the reciprocating piston engine with four pistons, a square-four arrangement can prove to be particularly advantageous if two pistons in each case are connected to the same crankshaft with a crank pin offset of 270° by connecting rods. In other words, a tandem arrangement of the cylinders is doubled, such that the aforementioned square-four arrangement is provided, wherein two cylinders per crankshaft are arranged in series in each case. A crankshaft with 270° crank pin offset does not generate any second-order mass forces, but instead second-order moments, which act in the cylinder axis plane. With the top dead centre being reached simultaneously by diagonally opposite pistons of the “square four”, the moments of both crankshafts therefore cancel out one another. There is thus no need for any further balancing measures for the second-order forces or moments.
The arrangement of the cylinders—both in tandem and in square-four form—can be parallel to one another or inclined relative to one another with an angle of inclination between 1° and 15°, preferably between 4° and 8°, per cylinder.
In a further variant of the invention it is provided that each cylinder is in fluid connection with an outlet channel, wherein the outlet channels of the cylinders open into a common central exhaust gas channel. The outlet channels can have identical lengths. The central exhaust gas channel is preferably arranged in an engine central plane. The engine central plane runs between the cylinders parallel to the crankshaft axes and is arranged at the same distance from each of the crankshaft axes. Due to this design of the outlet channels, the distance between the cylinders, in particular the outlet valves, and an auxiliary unit, for example a turbine of a turbocharger, advantageously can be made relatively short. A relatively large amount of the available enthalpy can thus be made available for the operation of the auxiliary unit. In addition, it is possible to do away with a conventional, relatively heavy exhaust manifold. This reduces the weight of the reciprocating piston engine. The complex connection of the exhaust manifold to the cylinder head can also be avoided, and the cylinder head is therefore constructed in a compact manner. An air-gap-insulated port liner can be arranged in the central exhaust gas channel
A turbocharger can be connected directly to the central exhaust gas channel. Said turbocharger is in this way operated with a high efficiency and increases the performance of the reciprocating piston engine.
The invention will be explained in greater detail hereinafter on the basis of exemplary embodiments with reference to the accompanying schematic drawings.